Session: 09-02-08 Wind Energy: System Testing

Submission Number: 176171

A Parameter Framework for Interpreting Scaled Mooring Experiments in Floating Offshore Wind Turbines 

Floating offshore wind turbines (FOWTs) have high potential to decarbonize the energy mix but still require further development to reach commercialization. Several potential wind farms are planned at 60–100 m water depth, such as Green Volt in Scotland and Korea Floating Wind in South Korea. These depths are relatively shallow from a mooring design perspective, leading to larger platform motions and higher sensitivity to mooring stiffness and damping characteristics. Over the past decade, numerous basin experiments have examined platform motions, hydrodynamic responses, and mooring loads under various configurations. However, these tests often differ in objectives, scale ratios, and simplifications, resulting in results that are difficult to compare. Consequently, many mooring characteristics, such as stiffness transitions and hydrodynamic damping contributions, remain insufficiently described. Physical modelling remains essential for design development and validation, yet scaled mooring experiments are typically case-specific with limited comparability and uncertain transferability to prototype conditions. Developing a consistent analytical framework would enable clearer identification of key physical mechanisms in each experiment and better assessment of their relevance to full-scale behaviour. 

To address this gap, a six-parameter framework is being developed as a practical toolbox to analyse and interpret scaled mooring experiments. The parameters encompass Froude number (Fr) that representing the ratio of inertial to gravitational forces, the Reynolds number (Re) describing the relative influence of inertia and viscosity, the Keulegan–Carpenter number (KC) which characterises the dominance of drag or inertia in oscillatory flow, the elastic wave speed ratio compares the propagation speed of axial tension waves along the mooring line to the imposed motion velocity, the added-mass ratio which expresses the hydrodynamic inertia relative to the line’s structural inertia and the weight-to-stiffness ratio quantifies the balance between submerged weight and axial stiffness that governs the static restoring behaviour. This framework aims to explicitly evaluate how each mechanism scales and whether it is preserved or relaxed in the chosen setup. This makes it possible to separate hydrodynamic similitude from structural and dynamic similitude, and to interpret deviations not as errors but as intentional compromises inherent to scaled testing. Progress in the development and preliminary application will be demonstrated using benchmark datasets from the OC4 DeepCwind definition [1], the INNWIND 1:45 model tests [2], Barrera’s catenary line experiments [3], and Hsu’s shallow-water snap load study [4], with graphical analyses employed. 

The advantage of this approach is that it transforms scattered case-specific observations into a quantitative methodology for comparison. Rather than treating scale effects as opaque, the framework makes their origin and impact transparent. This paper, therefore, presents ongoing progress toward a reusable diagnostic tool that will allow others to evaluate the credibility of past experiments and to design future basin campaigns with clear guidance on which similitudes to prioritize. 

[1] A. Robertson, J. Jonkman, M. Masciola, H. Song, A. Goupee, A. Coulling, and C. Luan, Definition of the Semisubmersible Floating System for Phase II of OC4, NREL Technical Report TP-5000-60601, National Renewable Energy Laboratory, Golden, CO, USA, 2014. 

[2] J. Azcona, F. Lemmer, D. Matha, F. Amann, C. L. Bottasso, P. L. Montinari, P. Chassapoyannis, K. Diakakis, S. Voutsinas, R. Pereira, H. Bredmose, R. Mikkelsen, R. Laugesen, and A. M. Hansen, INNWIND.EU Deliverable D4.2.4: Results of Wave Tank Tests, EU FP7 Project Report, 2016. 

[3] C. Barrera, R. Guanche, and Í. J. Losada, “Experimental modelling of mooring systems for floating marine energy concepts,” Marine Structures, vol. 63, pp. 153–180, 2019. https://doi.org/10.1016/j.marstruc.2018.08.003  

[4] W. Y. Hsu, T. C. Chuang, R. Y. Yang, W. T. Hsu, and K. P. Thiagarajan, “An experimental study of mooring line damping and snap load in shallow water,” Journal of Offshore Mechanics and Arctic Engineering, vol. 141, no. 2, 021701, 2019. https://doi.org/10.1115/1.4042535 

Presenting Author: Yuki Igarashi Ghent University

Presenting Author Biography: PhD candidate at Ghent University, Coastal Engineering Research Group. Expertise in floating offshore wind turbine mooring engineering, particularly in shallow water and extreme conditions. Conducting research that combines experimental and numerical approaches.

Authors:

Yuki Igarashi Ghent University
Annelie Baines-Maitland Ghent University
Maximilian Streicher Ghent University
Peter Troch Ghent University